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The Dipole Project: Towards High Energy, High Repetition Rate Diode Pumped Lasers

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The Dipole Project: Towards High Energy, High Repetition Rate Diode Pumped Lasers The DiPOLE project: towards high energy, high repetition rate diode pumped lasers Contact [email protected] Klaus Ertel, Saumyabrata Banerjee, Paul Mason, Jonathan Tristan Davenne, Michael Fitton, John Hill, Andrew Phillips, Phil Rice, Steph Tomlinson, Christian Sawyer, Lintern Steve Blake, Cristina Hernandez-Gomez, John Collier Engineering Technology Centre, STFC Rutherford Appleton Central Laser Facility, STFC Rutherford Appleton Laboratory, Laboratory, Chilton, Didcot, OX11 0QX, UK Chilton, Didcot, OX11 0QX, UK Introduction Gain material selection DiPOLE stands for Di ode Pumped Optical Laser for The starting point was the identification of the most suitable Experiments. It is a new project at the CLF to develop the gain material. This material needs to provide: foundations of novel high energy, high average power laser • A long fluorescence lifetime to minimise the number of systems based on diode pumped solid state laser (DPSSL) pump diodes required technology. Compared to conventional systems, this approach promises dramatically increased repetition rates (and hence • Good thermo-mechanical properties to handle the high average powers) at significantly higher electrical-to-optical average power conversion efficiency. DiPOLE has been included as an • Reasonably high gain cross section to enable uncomplicated emerging opportunity in the Research Councils UK Large and efficient energy extraction Facilities Roadmap [1]. • The possibility to be manufactured in large sizes to handle Motivation the high pulse energy. Laser amplifiers capable of producing energetic ns-pulses are Ytterbium (Yb) as an active laser ion offers very long one of the main tools for laser plasma research and high-energy fluorescence lifetimes, a low quantum defect (pump wavelength applications. Laser chains containing such amplifiers can 940 nm, laser wavelength 1030 nm), reasonable gain cross produce ns-pulses or ps-pulses if the chirped pulse amplification sections and efficient high power laser diodes are readily (CPA) technique is used. Depending on the application, these available for its pump wavelength. Two host materials have pulses are either applied directly or are used to pump other been identified that offer good thermo-mechanical properties amplifiers (e.g. Ti:Sapphire or OPCPA) in order to obtain even and can be manufactured in large sizes: crystalline calcium shorter pulses in the fs-regime. fluoride (CAF) and ceramic YAG. Currently, ns-amplifiers are based on flashlamp-pumped Yb:CAF has a very long fluorescence lifetime of 2.4 ms and a Nd:glass technology and their repetition rate is limited to a few large gain bandwidth (> 50 nm) [4], and is therefore a shots per minute for amplifiers delivering tens of joules of pulse promising candidate for directly diode pumped chirped pulse energy to a few shots per day for lasers delivering kJ-level pulse amplification (CPA) systems for producing sub-ps pulses [5]. energies. However, since it exhibits a very small gain cross section, very Increasing the repetition rate of such laser systems to the multi- large fluence levels are required for both pumping and Hz level (typically 10 Hz) is pivotal for the following extraction in order to achieve good optical-to-optical (o-o) applications: efficiency. • Opening up new horizons in fundamental laser plasma interaction research by enabling higher throughput and the exploration of larger parameter spaces. • So-called secondary sources which use laser-generated plasmas to produce ultra-short pulses of energetic particles (electrons or ions) or electromagnetic radiation (ranging from THz to hard X-ray). High repetition rate drive lasers are required to generate sufficiently high particle and photon numbers. Much of the pan-European ELI project focuses on the development and exploitation of secondary sources [2]. • Inertial confinement fusion (ICF), which is expected to be demonstrated for the first time within the next two years. Whereas current low-repetition rate facilities like NIF and Fig. 1: Yb:YAG - Cr 4+ :YAG compound disk. LMJ are suitable for proof of principle experiments, high- efficiency, high repetition rate DPSSL based laser drivers On the other hand, Yb:YAG has an order of magnitude higher open up the possibility to develop ICF into a reasonably gain cross section with a reasonable fluorescence lifetime of clean, practically inexhaustible source of energy. This is the 1 ms [6]. Since the main application of our envisioned kJ-class focus of the pan-European HiPER project [3]. laser is the production of ns-pulses, either for pumping amplifiers for fs-pulse generation (Ti:sapphire or OPCPA) or Amplifier concept for driving inertial fusion targets, we think that ceramic The main activity within DiPOLE is the development of a Yb:YAG is the best choice for the gain medium. Also, DPSSL amplifier concept that is capable of delivering kJ-level monolithic compound structures with different doping species pulses at 10 Hz repetition rate. are possible with ceramic YAG [7]. The photo of such a compound disk is shown in Fig. 1. Here the inner region of the therefore chosen as the preliminary operating point for our disk is doped with Yb 3+ and acts as the active laser medium, the amplifier. outer region is doped with Cr 4+ which heavily absorbs at 1030 nm and therefore acts as an index-matched absorber for suppression of amplified spontaneous emission (ASE) [8]. Efficiency and gain modelling Numerical modelling has been carried out to determine optimum amplifier design parameters. In the model, the storage efficiency ηstor has been calculated for various parameters like pump fluence, pump pulse duration and pump spectral width. ηstor is defined as extractable fluence divided by pump fluence. Loss mechanisms and associated efficiency factors that influence ηstor are: ηfluo for the fluorescence decay, ηQD for the quantum defect, ηabs for pump light that passes through the gain medium without being absorbed and, finally, ηreabs for the minimum upper state population that needs to be established in order to overcome reabsorption that exists due to the quasi-3- Fig. 3: Maximum storage efficiency (blue) and small signal level nature of Yb:YAG. If a pump pulse duration of 1 ms is gain (red) for amplifier operated at 175 K. chosen, η and η limit η to 58 %. It turns out that η QD fluo stor abs Operating at cryogenic temperatures yields the added benefit of and η need to be balanced off against each other and that for reabs improved thermo-mechanical and thermo-optical properties like a given set of pump-related parameters, there is one optimum increased thermal conductivity and reduced temperature value for gain medium optical depth (OD = thickness times dependence of the refractive index [10]. Another effect is the doping concentration) that yields the maximum η . If OD is stor narrowing of spectral features both in the emission and the chosen too low, too little pump light is absorbed, if OD is too absorption spectrum. The absorption spectra measured at room high, reabsorption losses become dominant. temperature and at 175 K are shown Fig. 4, together with a The following results are calculated, unless stated otherwise, for 5 nm FWHM Gaussian spectrum for comparison, which is an amplifier that is end-pumped from both sides with a pump assumed to be the spectral shape of our pump source. pulse duration of 1 ms, a 5 nm FWHM pump spectral width, centred at the optimum wavelength in the 940 nm absorption band. Spectrally resolved pump absorption cross sections were taken from [9]. Quantities calculated were ηstor and the small signal gain G, defined as G = exp( ηstor Fpump /F sat ) where ηstor Fpump is the extractable fluence and F sat the gain saturation fluence. First, calculations were carried out for room temperature operation. The results are shown Fig. 2. It becomes apparent that very strong pumping is required, firstly to overcome the high reabsorption losses and to achieve good efficiency and secondly to overcome the still rather low gain cross section and achieve reasonable gain. The required high pump and extraction fluences are difficult to achieve because of limited pump source brightness and limited laser damage threshold. Fig. 4: Absorption spectra of Yb:YAG at different temperatures and 5 nm wide pump diode spectrum for comparison. Fig. 2: Maximum storage efficiency (blue) and small signal gain (red) for amplifier operated at room temperature. Cooling the gain medium to 175 K drastically changes the Fig. 5: Storage efficiency as function of pump centre wave- situation, as illustrated in Fig. 3. Reabsorption is reduced and length for two different temperatures and pump fluences. the gain cross section increased, leading to greatly improved efficiency and gain, especially at moderate fluences. A pump If these pump and absorption spectra are used to calculate fluence that is realistically achievable with today’s laser diodes storage efficiency for two different temperature scenarios, is 10 J/cm 2 (5 J/cm 2 from each side), yielding a storage results as shown in Fig. 5 are obtained. Even though the pump efficiency of just over 50 % (resulting in an extractable fluence fluence in the low temperature scenario is only half that of the of 5 J/cm 2) and a small signal gain of 3.8. This fluence is room temperature case, significantly higher storage efficiency is predicted. Lower temperature operation also shows a much weaker dependence on pump centre wavelength. So despite DiPOLE prototype narrower absorption features, the requirements with respect to To test the concept in the laboratory a lower-energy multi-J spectral performance of the pump diodes are less critical for low prototype amplifier system is currently being built. The design temperature operation. is based on four co-sintered ceramic YAG discs (55 mm in Amplifier geometry diameter x 5 mm thick) where the Yb-doped region (35 mm diameter) is surrounded by a 10 mm thick Cr 4+ cladding to After determining operating temperature and pump fluence for absorb unwanted transverse fluorescence.
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